The subject of the invention is a device for the detection of variations, in the physical state during heating of the fusion-weldable plastic material in a coupling piece which is to be welded to another piece of the same material in the shape of a pipe with a circular or substantially circular cross-section.
Pipes made of plastic, especially of polyethylene, are being used more and more frequently for many purposes, particularly in gas distribution systems.
In practice, it is impossible to join such pipes to each other satisfactorily by gluing them together, so that welding, or more precisely fusion welding, is used for this purpose.
To achieve this, a coupling piece is used which mainly consists of a sleeve for joining together one of the ends of two pipes or alternatively, for example, of a locally saddle-shaped branching joint capable of the transverse coupling of the pipes. Whatever coupling piece is chosen, it usually incorporates an electrical heating resistor embedded in the material of which it is made and located near the surface that will come into contact with the surface of the pipe. The welding operation is carried out by connecting the resistor to a source of electricity (such as a rectified alternating current) which will cause Joule heating of the resistor and produce softening and melting of the surrounding plastic material, thus achieving the welding.
Although this method of connection is attractive, it does present the difficulty, of controlling the time for which the resistor needs to be heated in order to obtain a high quality weld and thus to form the pipe or pipes and the coupling piece into a solid block.
At present, it is known that such control can be satisfactorily achieved by monitoring two physical quantities: the temperature and the volume of the material of the pieces to be welded.
To achieve active monitoring of these quantities, it has been proposed that, in one of the pieces to be assembled, at least one cavity should be formed, into which the softened material penetrates. Such a cavity extends along an axis substantially radial with respect to the pipe or pipes until it is near a part of the resistor, where it ends in a bottom wall. With this cavity is associated a sensor capable of detecting variations in the physical state of the material within the cavity, the sensor being itself connected to means for commanding switching off of the electrical power supply to the heating resistor.
Various types of sensors have already been considered elsewhere, such as for example a thermal probe, a microswitch sensitive to the pressure of the softened material within the cavity, or else a sensor sensitive to the hardness or consistency of this same material.
Such sensors, and others, are mainly described in the French patent application FR-A-2,555,936 and in its certificates of addition FR-A-2,572,327 and FR-A-2,600,008.
However, in spite of all these improvements, certain problems still remain and the quality of the welding of the pieces is not always satisfactory.
In particular, various tests have shown that, for reduced coupling clearances between the pieces, the welding time has a tendency to decrease while the clearances increase, which, as can be appreciated, seriously degrades the quality of the weld.
In fact, the coupling clearances may in practice vary considerably, depending on the pieces being used.
Moreover, it is impossible with present-day welding techniques to obtain, under good conditions, a situation in which the thickness of the material melted during the heating increases with the size of the pieces to be assembled, which is in practice necessary.
The aim of the invention is to solve these various problems by defining conditions for the shape, size and position of the cavities into which the softened material penetrates during the heating and within which the sensors are located for recording the variations in its physical state.